The present invention relates to induction machines, and in particular to field oriented control (FOC) of induction machines.
Field Oriented Control (FOC) is a well-known method of controlling induction machines. In short, field oriented control transforms space vectors from a three-axis stationary reference frame (abc) to a two-axis rotating reference frame (dq). Field oriented control allows for precise control of induction machines. In particular, the d-q reference frame allows the rotor flux or direct component (d-axis component) and the torque or quadrature component (q-axis component) of a commanded current signal to be independently controlled. Thus, the rotor flux and torque produced in an induction machine can be precisely controlled.
A typical implementation of FOC uses transforms to convert from the three-axis stationary reference frame (abc) to the two-axis rotating reference frame (dq) (as applied to a three phase machine). The two-axis reference frame can be aligned with the rotor flux or, in the alternative, can be aligned with the stator flux or air gap flux. Aligning the rotating reference frame with the rotor flux allows the decoupling of the d-axis current (used for rotor flux production) and the q-axis current (used for torque production). This decoupling is the heart of FOC, and is accomplished by providing the induction machine with the correct slip frequency and stator currents (magnitude and angle).
The accuracy, effectiveness, and dynamic performance of a particular FOC scheme rest on the ability to correctly determine the required slip frequency and q-axis current vector for a desired torque and rotor flux command. Correctly determining these values depends in part on accurately estimating induction machine circuit parameters. The rotor time constant associated with the induction machine is of particular importance for these calculations and is expressed as rotor inductance divided by rotor resistance (Lr/Rr). Because resistance of the induction machine changes with the temperature of the rotor, it is not sufficient to use nominal parameters continuously for good dynamic performance. Furthermore, since the rotor is moving, it is difficult to obtain temperature measurements for direct compensation with temperature. Therefore, accurate estimation or measurement of the rotor time constant is an important aspect of field oriented control.